The present invention relates generally to semiconductor processing. Specifically, the present invention is directed to systems and methods for measuring the amount of liquid remaining in a bubbler ampule used for chemical vapor deposition (CVD) on a semiconductor wafer.
Prior to introduction into a CVD processing chamber, deposition material supplied to the processing chamber is typically in a liquid or gaseous state. In one method of introducing liquid material into a CVD processing chamber, a carrier gas is bubbled through an ampule containing the liquid, and the resulting gas mixture is directed towards the processing chamber. Through the course of wafer processing, the liquid chemicals in the ampule will eventually become depleted and require replacement.
In a typical semiconductor process, the total liquid chemical replacement process can take from two to eight hours, depending upon the chemical involved and the system configuration. A bubbler ampule with fresh chemicals is inserted in place of the depleted ampule which is returned to the chemical manufacturer who cleans and refills the depleted ampule for future use. Unfortunately, the entire CVD system is inoperable during this replacement time, and temperatures in the bubbler ampule and other portions of the CVD system are lowered during these periods of non-operation. Prior to restarting the process, both the bubbler ampule and other parts of the CVD system usually must be reheated to their operating temperatures. Also, test samples are routinely run through the process to ensure that the replenished chemical is not contaminated prior to resuming the production operation.
It is desirable to coordinate the replacement of depleted ampules so as to minimize the effect on wafer production. However, it is difficult to ascertain when a bubbler ampule needs replacing. Often, the depletion of liquid in the ampule may occur at nonlinear rates, requiring frequent monitoring of liquid levels. With each wafer costing in the thousands of dollars, significant financial losses may result from ruining a batch of wafers when deposition material is depleted during processing or from losing production time while waiting for a replacement ampule to arrive.
Unfortunately, the desire to measure the liquid remaining in an ampule is hampered by the deficiencies of conventional liquid level sensors. As discussed below, the nature of chemicals used in processes such as aluminum (Al) CVD severely limits the usefulness of these known sensors. Organoaluminum compounds such as dimethylaluminum hydride (DMAH) used in Al-CVD processes are liquid at room temperature, corrosive to many metals, and violently explosive when exposed to ambient air or water. Details on DMAH and Al-CVD methods can be found in R. Bhat et al., J. Crystal Growth, vol. 77 pp. 77 (1986), the complete disclosure of which is incorporated herein by reference. These qualities of organoaluminums present a variety of challenges for liquid level sensors.
First, to reduce the risk of accidentally exposing these chemicals to ambient air or water due to rupture or impact, the bubbler ampules for these organoaluminum compounds are typically made of materials, such as stainless steel, which will stretch and deform prior to breaking. Unfortunately, ampules made of such resilient materials are almost always non-transparent and do not provide visual cues as to the amount of liquid it contains. Further, the concern over leakage prevents the installation of conventional glass or other clear viewports to monitor levels of liquid remaining within the ampule. For this reason, nontransparent ampules make it difficult to determine how much liquid remains in the ampules, and this uncertainty may lead to significant losses in manufacturing time and material.
Second, the operating environment within the bubbler ampule interferes with the accuracy and reliability of other conventional sensors. Most optical sensors are too fragile to withstand transport and cleaning of the ampule when it is returned the chemical vendor. Metallic float sensors which typically slide along a vertical rod inside the ampule are unreliable as they are subject to corrosion which leave deposits that prevent the float from moving with the liquid level. Metallic float sensors also increase the risk of contaminating the high purity deposition material as they may shed metal particles or ion into the ampule environment as they slide against other metal parts in the ampule.
To avoid these drawbacks, some known liquid level sensors such as Advanced Process Technology's Liquid Level 2000 use gas pressure differentials to measure the depth of liquid remaining in a bubble ampule. The liquid level 2000 sensor is designed, however, to provide meaningful measurements only when a carrier gas is flowing through the sensor. This presents a problem in that the viscosity and other properties of organoaluminum compounds such as DMAH adversely affect the reliability and accuracy of the system. Further details of the liquid level 2000 sensor are discussed below with respect to FIG. 4B.
Accordingly, improved methods and devices are needed for reliably monitoring liquid level within an opaque container. Preferably these improved methods and devices will monitor the liquid level without contaminating the environment within the container and also provide desired levels of safety and accuracy during measurements.